348 H. M. BKRNAUD. 



matin, or else nuclei in which the chromatin has contracted to 

 a dense refractive mass in its centre. 



Such stages are very marked in the " ganglionic " layer, 

 for here the vesicular nuclei attain sizes reached in no other 

 laj^er. The contrasts between the phases in this layer are some- 

 times very great (see figs. 2 c, and 3 a, b). I have measured 

 many hundreds of these contrasting nuclei in this layer, and 

 have found them range between as much as 18 fx and 4 fx. 

 The contracted nuclei are frequently associated with fluid 

 spaces in the inner reticular layer, as if fluid had just been 

 discharged from them. I was, indeed, at one time inclined 

 to think that this propulsion of fluid was one of the chief 

 functions of the nuclei of this layer. But I soon noticed that 

 these phases were to be met Avith through all the layers of 

 the retina. 



Further, it is to be noted that these phases of the nuclei are 

 sometimes found in zones. These zones are naturally most 

 marked in the middle layer, because in that there are most 

 nuclei. In this layer almost every observer has noted that 

 the innermost of its nuclei are usually large and vesicular, 

 while the outer are smaller, and with denser chromatin. These 

 zones are, however, not homogeneous, for among the vesicular 

 nuclei others which are contracted can always be found, while, 

 among the contracted, others in the vesicular stage occur. 

 But it is not only in the middle layer that this is marked. 

 Figs. 22 h, c, Part V, show parts of the section of a plaice in 

 which the nuclei of all the layers show zonal arrangement. 

 In the two inner layers, the vesicular nuclei are again on the 

 proximal side, but, in the layer of rod-nuclei, aline of vesicu- 

 lar nuclei runs along the middle of the layer, with contracted 

 n\iclei on both its inner and outer sidi^s. hlxcept in this 

 outermost layer the zones are not sliiirjily marked off, but 

 there is a good deal of mixing. It is ditflcult to account for 

 such a regular zonal arriingenient of the two kinds of nuclei 

 (vesicular and contracted), except on the assumption of a 

 series of waves of pulsation passing through the retina, the 

 phases of which are fixed in the preparation. 



